1. Field of the Invention
The present invention relates to photonic bandgap optical fibers, and more particularly, to photonic bandgap optical fibers capable of guiding light of multiple bands by use of a photonic crystal structure forming plural band-gaps.
2. Description of the Related Art
In general, optical fibers consist of a core of glass or plastic material having a higher refractive index inside a clad of glass or plastic material having a lower refractive index. Light propagates along the inner core due to total internal reflection, by which information can be sent. The optical fibers are so light-weighted and thin that they do not take up much space, but provide a high transmission speed and a low error rate. Thus, optical fibers are widely used in data transmission fields requiring a high rate of data transmission and reception and high reliability. However, in optical fibers light propagates through a solid core so that the core material causes scattering, dispersion, or absorption loss, which limit their efficiency.
Thus, photonic band-gap optical fibers which guide light by means of photonic band-gaps are of more interest even though the refractive index of the core is lower than that of the clad. In a photonic band-gap optical fiber, a photonic crystal structure is used to form photonic band-gaps. The photonic crystal structure has dimensions on the order of a few hundred nm to a few thousand μm depending on the frequency band to be used, and has substances with different refractive indexes that are regularly arranged. The photonic crystal is used to form a complete band-gap that does not pass polarized light irrespective of its incident angle, and further is used to form an absolute band-gap that passes no light independent of polarization. Due to such characteristics, the photonic crystal can be used to fabricate optical devices such as split filters, optical waveguides, optical delay devices, and optical fibers.
On the other hand, the photonic crystal has three kinds of structures such as one dimensional, two dimensional, and three dimensional structures, depending upon the number of periodic orientations. A variety of detailed structures have been proposed for all dimensions. For example, if an appropriate structure is selected for a two-dimensional photonic crystal, an absolute band-gap can be formed that prevents light having a wavelength about two times longer than a grid constant from propagating in any direction within the periodic structure. The characteristics of such a photonic crystal are determined by factors such as grid shape, grid constant, shape of an inserted pile, and the like. Further, if a round pile is filled, the characteristics of the photonic crystal are determined by its radius, permittivity of a background material, permittivity of the filled pile, and the like.
FIG. 1 is a view showing a two-dimensional photonic crystal structure. FIG. 1 depicts a photonic crystal whose cross-section is two dimensional (having two dimensions in the x-y cross-section), and where second media 15 of a cylindrical shape of radius of R having a second permittivity are periodically arranged in the first medium 13 having a first permittivity. The relationships between wave vectors and frequencies for the photonic crystal structure of FIG. 1 can be obtained by solving the Maxwell equations.
FIG. 2 is a view showing band-gaps for the photonic crystal structure of FIG. 1 having a grid constant p and a propagation vector β in a z-direction which is a light propagation direction. Here, silica having a refractive index of 1.45 is used for the first medium 13 and the filling factor is 0.7.
FIG. 3 is a cross-sectioned view showing a conventional photonic band-gap optical fiber. As shown in FIG. 3, a conventional photonic band-gap optical fiber 30 has a first medium 33 having a first permittivity and a second medium 35 of cylindrical shape arranged in the first medium 33 using the photonic crystal shown in FIG. 1. Further, the photonic band-gap optical fiber 30 has a local hollow portion 37 formed in its center which breaks or rather interrupts the periodic arrangement of the second medium 35. The local hollow portion 37 formed in the center forms a defect mode so that light can propagate within the center of the fiber.
FIG. 4 is a view showing a propagation mode of the conventional photonic band-gap optical fiber of FIG. 3. As shown in FIG. 4, light is concentrated about the center of the photonic band-gap optical fiber. Light, excepting the center, is blocked due to the band-gap, which can be seen by measuring the intensity of an electric field. Thus, light can propagate through such a photonic bandgap optical fiber, and the frequency band can be wider as compared with general optical fibers.
However, as mentioned above, the conventional photonic band-gap optical fiber is based on a photonic crystal structure having one band-gap for a specific propagation constant as a scale parameter is varied. Therefore, in a conventional photonic band-gap optical fiber, plural modes of light of different bands can not propagate at the same time. This is because its fundamental mode which allows light to propagate exists only in one band region.